A primary focus and application of the present invention is the field of radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to the antenna being significantly lower than the maximum possible power, leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high-power efficient topologies capable of providing useful performance in the ‘back-off’ (linear) region of the power amplifier.
Linear modulation schemes require linear amplification of the modulated signal in order to minimise undesired out-of-band emissions from spectral re-growth. However, the active devices used within a typical RF power amplifier are inherently non-linear by nature. Only when a small portion of the consumed direct current (DC) power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier. This mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, low efficiency is also increasingly being recognised as being problematic for base stations.
Additionally, the emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearisation techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearising techniques exist, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion, which are often used when designing linear transmitters.
Voltages at the output of the linear, e.g. Class AB, amplifier are typically dictated by the requirements of the final RF power amplifier (PA) device. Generally, the minimum supply voltage of the PA is significantly larger than that required by the output devices of the Class AB amplifier. Hence, they are not the most efficient of amplification techniques. The efficiency of the transmitter (primarily the PA) is determined by the voltage across the output devices, as well as any excess voltage across any pull-down device components due to the minimum supply voltage (Vmin) requirement of the PA.
In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-bit quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity.
To help overcome such efficiency and linearity issues a number of solutions have been proposed.
To raise efficiency of a transmitter that uses a linear PA, the linear PA is supplied with a time-varying voltage that tracks an envelope of a signal to be transmitted, typically referred to as ‘envelope tracking’. A known supply voltage technique 100 is to combine envelope tracking (ET) with digital pre-distortion (DPD), as illustrated in FIG. 1. FIG. 1 illustrates modulating the radio frequency (RF) power amplifier (PA) supply voltage (Vpa) 120 to match (e.g. track) the envelope of the radio frequency waveform being transmitted by the RF PA 140. Careful control of the supply voltage helps in maximizing PA efficiency for each PA output power, in that the closer to this value the supply voltage is, the higher PA efficiency is. Typically, ET systems control the RF PA supply voltage 120 in order to improve PA efficiency through selecting a lower supply voltage dependent upon an instantaneous envelope of the input signal, and to improve linearity by selecting a RF PA supply voltage 120 dependent upon a constant PA amplification gain.
In the illustrated supply voltage technique 100, control/manipulation of the input waveform/signal in the digital domain is performed in order to compensate for PA nonlinearity (AM-to-AM and AM-to-PM) effects, thereby improving PA output linearity based on prior information or measured data of the PA system. A digital (quadrature) input signal 102 is input to a digital pre-distortion (DPD) function 130. The DPD function 130 pre-distorts the input digital signal, such that the non-linear effects subsequently caused by the PA 140 can be pre-compensated. The output 132 from the DPD function 130 is input to RF transmitter 134, whose output provides an input power level 136 to the RF PA 140. The RF PA output 122 is typically output to an antenna and/or matching circuit.
Concurrently, the digital (quadrature) input signal 102 is applied to an envelope detector 104 arranged to determine a real-time envelope of the signal to be transmitted (e.g. radiated). The determined real-time envelope signal output from the envelope detector 104 is input to an envelope mapping function 110, which is arranged to determine a suitable PA supply voltage (Vpa) 120 to be applied to the PA 140 to substantially match the instantaneous real-time envelope of the signal to be transmitted. The output from the envelope mapping function 110 is input to a supply modulator 114 that provides the PA supply voltage (Vpa) 120 to be applied to the PA 140.
With ET, the instantaneous PA supply voltage (Vpa) 120 of the wireless transmitter is caused to approximately track the instantaneous envelope (ENV) of the transmitted RF signal. Thus, since the power dissipation in the PA 140 is proportional to the difference between its supply voltage and output voltage, ET may provide an increase in PA efficiency, reduced heat dissipation, improved linearity and increased maximum output power 122, whilst allowing the PA to produce the intended RF output. However, the total system efficiency is affected by supply modulator efficiency, which is related to the supply modulator design, supply voltage range, bandwidth and PA loading. As the envelope bandwidth becomes wider, a supply modulator that uses a linear amplifier to fulfill the bandwidth requirement is less efficient. This typically results in the ET modulator efficiency not being high enough for most applications.
U.S. Pat. No. 6,788,151 B2 describes a variable output power supply for use in a linear amplification system.
There exists a need for a more efficient and cost effective solution to improve PA efficiency, particularly for wider bandwidth applications.